From the circuit diagram in this earlier thread: https://www.mig-welding.co.uk/forum/threads/snap-on-minimig-180c-wont-start-got-nothing.15762/
it looks like the auxiliary transformer has two secondary windings - one is 17v and the other is 42v.
Is the primary defective, or one secondary? The 17v supply powers the wirefeed motor, It looks like the 42v only runs a fan?
An original part with both secondaries will be hard to find. It is probably going to be easiest to get a transformer that can provide one of these outputs.